In both animal proof of concept studies and preliminary clinical trials, there is ample data demonstrating the potential therapeutic benefits o CD154 blockade for treatment of autoimmune diseases and organ transplantation. However, development of ?CD154 as a therapeutic has been impeded by antibody toxicity observed in early clinical trials. With nearly 50 million people suffering from autoimmune diseases, in the US alone, and with treatment costs projected to be over $100B/yr, development of safe, effective therapies is an utmost priority. Worldwide 2.5 million people are affected by MS, with 1/5 of patients found in the US. It is clear if toxicity can be resolved, and efficacy sustained, ?CD154 i a viable and attractive therapeutic that offers great promise for the treatment of autoimmune diseases. We will target MS as our first clinical indication for commercial development Existing studies strongly suggest that domains within the Fc region of the ?CD154 mAb contribute to its toxicity and therapeutic capacity. When toxicity was observed in the clinic and retrospectively in NHP, modifications were made to the antibody; while these modifications eliminated toxicity in NHP, the efficacy of ?CD154 as a tolerogenic antibody also was significantly reduced. As a result, development programs for ?CD154 as a therapeutic stalled. The goal of this proposal is to generate variant forms of the ?murine CD154 antibody that will retain the beneficial tolerogenic effects of aCD154 while greatly reducing or eliminating toxicity. Variant forms of the antibody will be evaluated in vivo using mouse models of transplantation, cell and humoral immunity, as well as assessing their toxicity. Successful proof of concept will be the basis for creating a similar variant of the human antibody that will be developed as a novel therapeutic, which could have far-reaching impacts on the treatment of autoimmune diseases and organ transplantation. PUBLIC HEALTH RELEVANCE: Development of aCD154 as a therapeutic for autoimmune diseases and transplantation has shown great promise in human clinical trials but development has been hindered by problems with antibody toxicity. In the past, when toxicity was reduced, the efficacy of the antibody was dramatically decreased leading to the suspension of developmental programs. This project will engineer new variants of the aCD154 antibody that will eliminate toxicity while maintaining tolerogenicity, thus allowing the potential of aCD154 to be harnessed as a novel treatment for a wide range of diseases.